The present invention relates to a variably operated valve system for an intake valve or an exhaust valve of a reciprocating engine and, more particularly, relates to the variably operated valve system suitable for use in a compression ignition engine such as a four-stroke and two-stroke Diesel engine.
Recently, a variably operated valve system which varies a lift and open and closure timings of the intake valve(s) and/or the exhaust valve(s) in accordance with an engine driving condition has widely been utilized in order to control a charge efficiency of the engine, an effective compression ratio, and a residual gas quantity and to improve an engine performance and an emission performance. In a Diesel engine or a pre-mixture compression ignition engine, due to a temperature rise of gas generated along with a compression stroke of the engine, an injected fuel is self ignited. The self-ignition of fuel is carried out only under such a condition that an in-cylinder temperature is high and a pressure is high. Although the self-ignition is dependent on a kind of fuel, the self-ignition is not carried out unless the temperature is equal to or higher than 1000 K and the pressure is equal to or higher than 1 MPa. Hence, during a low temperature start of the engine (so-called, a cold start), a temperature of a cylinder wall is low and a cylinder is deprived of a heat of gas. Unless a compression ratio is increased to be equal to or higher than, for example, 15 to increase the gas temperature and pressure within the cylinder, the self-ignition is not developed and a combustion of fuel cannot be achieved. However, at a time point at which a warm-up of the engine is completed, the high compression ratio causes the pressure acted upon a cylinder piston to be increased. Thus, a mechanical friction loss is increased and the engine performance is easy to be reduced. In order to avoid this, after the completion of the engine start, it has been proposed that the compression ratio is reduced to be equal to or lower than 15 to improve the engine performance. After the engine start, the temperature on the cylinder wall becomes higher. Even if the compression ratio is low, the cylinder is not deprived of the gas heat. Hence, the gas temperature and pressure become high and the self-ignition is carried out. The variation in the compression ratio is, as well known, carried out by mechanically changing a clearance volume of the piston or by mechanically changing a piston stroke. However, these mechanisms become complex. Wheread, a valve closure timing of the intake valve (or intake valves per cylinder) is retarded or advanced with respect to a crank angle so that a gas mass at a time of a compression stroke start can be varied. Rises in the gas pressure and temperature with respect to the crank angle can be retarded. That is to say, the effective compression ratio can be reduced. A previously proposed variably operated valve system is exemplified by a Japanese Patent Application First Publication No. Heisei 1-315631 published on Dec. 20, 1989 in which, in a two-stroke Diesel engine, an electrically driven variably operated apparatus (can twist type) makes an intake valve closure timing (IVC) approach to a bottom dead center (BDC) to increase the effective compression ratio. Thereby, the self-ignition during the engine start is assured and makes IVC (intake valve closure) retard during an ordinary driving so that the effective compression ratio is reduced and a fuel economy is reduced. In addition, another previously proposed variably operated valve mechanism is included in a rotary vane operated by means of a hydraulic as disclosed in a Japanese paper titled Recent trends in variable valve actuation technologies to reduce the emission and improve the fuel economy, pages 33 to 38 of Automotive Technology vol. 59, No. 2 by Yuuzou AKASAKA and Hajime MIURA.